Active matrix (AM) and other types of LCDs are increasingly being used as display devices in aircraft. To accommodate low level night operation and high ambient sunlight conditions, an extremely large backlight dimming range is necessary. A desired luminance range may be as large 0.03 FtL to 200 FtL (6667:1) over the display field of view. This is a much larger range than previously exhibited by typical hot cathode or cold cathode fluorescent lamps.
Previous backlight designs for backlighting LCDs have exhibited a limited luminance range of only Approximately 2000:1 for hot cathode lamps and 500:1 for cold cathode lamps. Conventional backlight designs cannot typically achieve the low end of the luminance range without the fluorescent lamp extinguishing or flickering. Since the difficulty in providing the extremely large dimming range occurs primarily at very low luminance levels, a backlight design capable of achieving a very low luminance uniform output without significantly affecting the maximum luminance level would be a significant improvement.
Additionally, transflective type LCDs have the disadvantage of high losses of luminance from the backlight device during night mode operations. Conventional transflective LCDs utilize a transflector which may reflect approximately 50 to 80 percent of ambient light impinging on the transflector's front surface. Thus, during the day with high levels of ambient light available, the reflected light can be used as a source of light to achieve sufficient daylight contrast, often without the use of the backlight. However, during night mode operation in which ambient light is generally unavailable for use in achieving desired contrast levels, a backlight must be used. Since conventional transflectors transmit only approximately 20 to 50 percent of the light generated by the backlight, they have proven to be inefficient during night mode operation. A transflective LCD, the transflector of which reflects a high percentage of ambient light for use in achieving high contrast during day mode operation, and which transmits a high percentage of backlight luminance to achieve high contrast during night mode operation, would also be a significant improvement.